The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Aluminum has been used for many years as a support for lithographic printing plates. In order to prepare the aluminum for such use, it is typical to subject it to both a graining process and a subsequent anodizing process. The graining process serves to improve the adhesion of the subsequently applied radiation-sensitive coating and to enhance the water-receptive characteristics of the background areas of the printing plate. The graining affects both the performance and the durability of the printing plate, and the quality of the graining is a critical factor determining the overall quality of the printing plate. A fine, uniform grain that is free of pits is essential to provide the highest quality performance.
Both mechanical and electrolytic graining processes are well known and widely used in the manufacture of lithographic printing plates. Optimum results are usually achieved through the use of electrolytic graining, which is also referred to in the art as electrochemical graining or electrochemical roughening, and there have been a great many different processes of electrolytic graining proposed for use in lithographic printing plate manufacturing. Processes of electrolytic graining are described, for example, in U. S. Pat. Nos. 3,755,116, 3,887,447, 3,935,080, 4,087,341, 4,201,836, 4,272,342, 4,294,672, 4,301,229, 4,396,468, 4,427,500, 4,468,295, 4,476,006, 4,482,434, 4,545,875, 4,548,683, 4,564,429, 4,581,996, 4,618,405, 4,735,696, 4,897,168 and 4,919,774.
In the manufacture of lithographic printing plates, the graining process is typically followed by an anodizing process, utilizing an acid such as sulfuric or phosphoric acid, and the anodizing process is typically followed by a process which renders the surface hydrophilic such as a process of thermal silication or electrosilication. The anodization step serves to provide an anodic oxide layer and is preferably controlled to create a layer of at least 0.3 g/m.sup.2. Processes for anodizing aluminum to form an anodic oxide coating and then hydrophilizing the anodized surface by techniques such as silication are very well known in the art, and need not be further described herein.
Included among the many patents relating to processes for anodization of lithographic printing plates are U.S. Pat. Nos. 2,594,289, 2,703,781, 3,227,639, 3,511,661, 3,804,731, 3,915,811, 3,988,217, 4,022,670, 4,115,211, 4,229,266 and 4,647,346. Illustrative of the many materials useful in forming hydrophilic barrier layers are polyvinyl phosphonic acid, polyacrylic acid, polyacrylamide, silicates, zirconates and titanates. Included among the many patents relating to hydrophilic barrier layers utilized in lithographic printing plates are U.S. Pat. Nos. 2,714,066, 3,181,461, 3,220,832, 3,265,504, 3,276,868, 3,549,365, 4,090,880, 4,153,461, 4,376,914, 4,383,987, 4,399,021, 4,427,765, 4,427,766, 4,448,647, 4,452,674, 4,458,005, 4,492,616, 4,578,156, 4,689,272, 4,935,332 and European Patent No. 190,643.
The result of subjecting aluminum to an anodization process is to form an oxide layer which is porous. Pore size can vary widely, depending on the conditions used in the anodization process, but is typically in the range of from about 0.1 to about 10 micrometers. The use of a hydrophilic barrier layer is optional but preferred. Whether or not a barrier layer is employed, the aluminum support is characterized by having a porous wear-resistant hydrophilic surface which specifically adapts it for use in lithographic printing, particularly in situations where long press runs are required.
A wide variety of radiation-sensitive materials suitable for forming images for use in the lithographic printing process are known. Any radiation-sensitive layer is suitable which, after exposure and any necessary developing and/or fixing, provides an area in imagewise distribution which can be used for printing.
Useful negative-working compositions include those containing diazo resins, photocrosslinkable polymers and photopolymerizable compositions. Useful positive-working compositions include aromatic diazooxide compounds such as benzoquinone diazides and naphthoquinone diazides.
Lithographic printing plates of the type described hereinabove are usually developed with a developing solution after being imagewise exposed. The developing solution, which is used to remove the non-image areas of the imaging layer and thereby reveal the underlying porous hydrophilic support, is typically an aqueous alkaline solution and frequently includes a substantial amount of organic solvent. The need to use and dispose of substantial quantities of alkaline developing solution has long been a matter of considerable concern in the printing art.
Efforts have been made for many years to manufacture a printing plate which does not require development with an alkaline developing solution. Examples of the many patents and published patent applications relating to such prior efforts include:
(1) Brown et al, U.S. Pat. No. 3,506,779, issued Apr. 14, 1970.
This patent describes a process in which a printing plate blank is imagewise exposed with a laser beam which is intensity modulated and deflected in accordance with control signals. The exposed areas are vaporized, thereby forming ink transferring recesses for intaglio printing or leaving raised ink transferring surfaces for letter press printing, or chemically altered to facilitate further processing.
(2) Caddell, U.S. Pat. No. 3,549,733, issued Dec. 22, 1970.
This patent describes a method for producing a printing plate in which a polymeric surface layer is subjected to a controlled laser beam of sufficient intensity to decompose the layer and form depressions in the surface of the plate.
(3) Burnett, U.S. Pat. No. 3,574,657, issued Apr. 13, 1971.
This patent describes a method for producing a printing plate in which an image is formed by exposing a cured allylic resin coating to a heat pattern.
(4) Mukherjee, U.S. Pat. No. 3,793,033, issued Feb, 19, 1974.
This patent describes a lithographic printing plate comprising a support and a hydrophilic imaging layer comprising a phenolic resin, an hydroxyethylcellulose ether and a photoinitiator. Upon imagewise exposure, the imaging layer becomes oleophilic in the exposed areas while remaining hydrophilic in the unexposed areas and thus can be used on a lithographic printing press, utilizing conventional inks and fountain solutions, without the need for a development step and consequently without the need for a developing solution.
(5) Barker, U.S. Pat. No. 3,832,948, issued Sep. 3, 1974.
This patent describes a method for producing a printing plate in which a surface in relief is formed by scanning coherent radiation over the surface of a radiation-absorptive thin film supported by a plastic substrate.
(6) Landsman, U.S. Pat. No. 3,945,318, issued Mar. 23, 1976.
This patent describes a method in which a lithographic printing plate blank is processed by applying a beam of laser radiation through a radiation transparent sheet to transfer selected portions on the sheet onto a lithographic surface.
(7) Eames, U.S. Pat. No. 3,962,513, issued Jun. 8, 1976.
This patent describes a method for producing a printing plate in which a transfer film comprising a transparent substrate, a layer comprising particles which absorb laser energy, and a layer of ink receptive resin is exposed with a laser beam to effect transfer to a lithographic surface.
(8) Peterson, U.S. Pat. No. 3,964,389, issued Jun. 22, 1976.
This patent describes a method for producing a printing plate in which a transfer film comprising a transparent substrate and a layer comprising particles which absorb laser energy is exposed with a laser beam to effect transfer to a lithographic surface.
(9) Uhlig, U.S. Pat. No. 4,034,183, issued Jul. 5, 1977.
This patent describes a lithographic printing plate comprising a support and a hydrophilic imaging layer that is imagewise exposed with laser radiation to render the exposed areas oleophilic and thereby form a lithographic printing surface. The printing plate can be used on a lithographic printing press employing conventional inks and fountain solutions without the need for a development step. If the hydrophilic imaging layer is water-insoluble, the unexposed areas of the layer serve as the image background. If the hydrophilic imaging layer is water-soluble the support which is used must be hydrophilic and then the imaging layer is removed in the unexposed areas by the fountain solution to reveal the underlying hydrophilic support.
(10) Caddell et al, U.S. Pat. No. 4,054,094, issued Oct. 18, 1977.
This patent describes a lithographic printing plate comprised of a support, a polymeric layer on the support, and a thin top coating of a hard hydrophilic material on the polymeric layer. A laser beam is used to etch the surface of the plate, thereby rendering it capable of accepting ink in the etched regions and accepting water in the unetched regions.
(11) Pacansky, U.S. Pat. No. 4,081,572, issued Mar. 28, 1978.
This patent describes printing plates comprising a substrate and a coating of a hydrophilic polymer containing carboxylic acid functionality which can be selectively imagewise converted to a hydrophobic condition by heat.
(12) Kitajima et al, U.S. Pat. No. 4,334,006, issued Jun. 8, 1982.
This patent describes a method for forming an image in which a photosensitive material composed of a support and a layer of a photosensitive composition is exposed and developed by heating in intimate contact with a peeling development carrier sheet and subsequently peeling the carrier sheet from the photosensitive material.
(13) Schwartz et al, U.S. Pat. No. 4,693,958, issued Sep. 15, 1987.
This patent describes a lithographic printing plate comprising a support and a hydrophilic water-soluble heat-curable imaging layer which is imagewise exposed by suitable means, such as the beam of an infrared laser, to cure it and render it oleophilic in the exposed areas. The uncured portions of the imaging layer can then be removed by merely flushing with water.
(14) Fromson et al, U.S. Pat. No. 4,731,317, issued Mar. 15, 1988.
This patent describes a lithographic printing plate comprising a grained and anodized aluminum substrate having thereon a coating comprising a diazo resin in admixture with particulate energy-absorbing material that will absorb incident radiation and reradiate it as radiation that will change the diazo resin coating.
(15) Hirai et al, U.S. Pat. No. 5,238,778, issued Aug. 24, 1993.
This patent describes a method of preparing a lithographic printing plate utilizing an element comprising a support having thereon a heat transfer layer containing a colorant, a heat-fusible substance and a photo-curable composition. Heat is applied in an image pattern to transfer the image onto a recording material having a hydrophilic surface and the transferred image is exposed to actinic radiation to cure it.
(16) Lewis et al, U.S. Pat. No. 5,353,705, issued Oct. 11, 1994.
This patent describes lithographic printing plates, suitable for imaging by means of laser devices which ablate one or more layers, which include a secondary ablation layer that ablates only partially as a result of destruction of overlying layers.
(17) Lewis et al, U.S. Pat. No. 5,385,092, issued Jan. 31, 1994.
This patent describes lithographic printing plates intended to be imaged by means of laser devices that emit in the infrared region. Both wet plates that utilize fountain solution during printing and dry plates to which ink is applied directly are described. Laser output either ablates one or more layers or physically transforms a surface layer whereby exposed areas exhibit an affinity for ink or an ink-abhesive fluid, such as fountain solution, that differs from that of unexposed areas.
(18) Reardon et al, U.S. Pat. No. 5,395,729, issued Mar. 7, 1995.
This patent describes a laser-induced thermal transfer process useful in applications such as color proofing and lithography. In this process, an assemblage comprising a donor element and a receiver element is imagewise exposed to laser radiation, the donor element is separated from the receiver element, and the receiver element is subjected to a post-transfer treatment to substantially eliminate back-transfer.
(19) European Patent Application No. 0 001 068, published Mar. 21, 1979.
This patent application describes a process for preparing a lithographic printing plate by providing an aluminum substrate having a hydrophilic porous anodic oxide layer thereon and depositing an oleophilic image in and on the porous layer by sublimation.
(20) European Patent Application No. 0 573 091, published Dec. 8, 1993.
This patent application describes a lithographic printing plate comprising a support having an oleophilic surface, a recording layer that is capable of converting laser beam radiation into heat, and an oleophobic surface layer. The recording layer and the oleophobic surface layer can be the same layer or separate layers. The printing plate is imagewise exposed with a laser beam and is then rubbed to remove the oleophobic surface layer in the exposed areas so as to reveal the underlying oleophilic surface and thereby form a lithographic printing surface.
Lithographic printing plates designed to eliminate the need for a developing solution which have been proposed heretofore have suffered from one or more disadvantages which have limited their usefulness. For example, they have lacked a sufficient degree of discrimination between oleophilic image areas and hydrophilic non-image areas with the result that image quality on printing is poor, or they have had oleophilic image areas which are not sufficiently durable to permit long printing runs, or they have had hydrophilic non-image areas that are easily scratched and worn, or they have been unduly complex and costly by virtue of the need to coat multiple layers on the support.
The lithographic printing plates described hereinabove are printing plates which are employed in a process which employs both a printing ink and an aqueous fountain solution. Also well known in the lithographic printing art are so-called"waterless" printing plates which do not require the use of a fountain solution. Such plates have a lithographic printing surface comprised of oleophilic (ink-accepting) image areas and oleophobic (ink-repellent) background areas. They are typically comprised of a support, such as aluminum, a photosensitive layer which overlies the support, and an oleophobic silicone rubber layer which overlies the photosensitive layer, and are subjected to the steps of imagewise exposure followed by development to form the lithographic printing surface.
It is toward the objective of providing an improved method of lithographic printing that requires no alkaline developing solution, that is simple and inexpensive, and which overcomes many of the limitations and disadvantages of the prior art that the present invention is directed.